A technique is provided which is capable of completing SOx poisoning recovery control on a NOx storage reduction catalyst in a shorter period of time. In the course of the SOx poisoning recovery control, the degree of decrease of a SOx release speed resulting from a decrease in an amount of SOx occlusion of the NOx catalyst (the degree of decrease of the SOx release speed (Ddw)) with respect to that at the time of start of the SOx poisoning recovery control is estimated. In accordance with the increasing degree of decrease of the SOx release speed (Ddw), a catalyst bed temperature (Tc) is caused to rise up to a higher side target temperature (Tct), and at the same time an exhaust gas air fuel ratio (A/F) is caused to lower up to a richer side target air fuel ratio (A/Ft), whereby the SOx release speed Vsox during the SOx poisoning recovery control is suppressed from lowering.